The invention is an improvement on various forms of known heat transmission measuring instruments (for instance, see Published, Unexamined German Patent applications No. DE-OS 24 47 617, 26 29 051, 32 13 902 and 32 22 046; also see copending application Ser. No. 07/172,457, now U.S. Pat. No. 4,856,329). In these heat transmission measuring instruments, hereinafter also called flow monitors, a differential temperature measurement is used. A first temperature measuring element measures a temperature (henceforth the "measured temperature") determined by the heating element and by the flowing medium, while a second temperature measuring element measures a temperature (henceforth the "reference temperature") that is determined by the flowing medium and if at all possible is not determined by the heating element or that is determined in some other manner. Accordingly the first temperature measuring element, a heating element and the second temperature measuring element are functionally necessary for flow monitors of this as is an evaluation circuit, the latter of type, which is of no further interest here.
If the medium the flow of which is to be monitored is a gas, the functionally necessary components--that is, the first temperature measuring element, the heating element and the second temperature measuring element--are introduced into the gas stream more or less unprotected. If the flow of a liquid medium is to be monitored, however, then the functionally necessary components--namely the first temperature measuring element, the heating element and the second temperature measuring element--cannot simply be introduced into the flow of liquid; instead, these components must be protected inside a measuring instrument housing.
In a flow monitor of the above type already used in industry, the measuring instrument housing is stepped; the first temperature measuring element and the heating element are located in a thinner part of the housing, while the second temperature measuring element is in a thicker part of the housing, on its end oriented toward the thinner part. The first temperature measuring element and the heating element are disposed in an aluminum disk; that is, the aluminum disk represents the thermal connection between the heating element and both the face end of the housing and the first temperature measuring element. The first temperature measuring element, the heating element and the aluminum disk are potted with an epoxy resin that is a good thermal conductor. The second temperature measuring element is spaced apart by a distance of approximately 1 cm from the face end of the measuring instrument housing, and is likewise potted with an epoxy resin that is a good thermal conductor. Because the heat of the heating element is not intended to reach the second temperature measuring element, material that is a good thermal insulator is located between the region in which the first temperature measuring element and the heating element are disposed and the region in which the second temperature measuring element is disposed.
The above-described flow monitor has significant disadvantages. First, it is thermally relatively sluggish; and second, it can be manufactured only at relatively high cost. However, the problem of how to construct a flow monitor of the type in question, which can react thermally relatively quickly and can be manufactured less expensively, has already been solved (see copending application Ser. No. 07/172,457) by providing that the first temperature measuring element, the heating element and the second temperature measuring element are applied to one side of a substrate foil that is a good electrical insulator and a good thermal conductor, and the substrate foil, on its side remote from the temperature measuring elements and the heating element, at least in the regions in which the temperature measuring elements and the heating element are applied, is put into thermally conductive contact with the measuring instrument housing. Preferably, the substrate foil is of polyimide, and has a thickness of approximately 50 to 150 .mu., in particular approximately 75 .mu.. A substrate foil of this kind has extremely low thermal resistance and is a good electrical insulator; the dielectric breakdown voltage of such a substrate foil is many thousands of volts.
Because in the previously described flow monitor the functionally necessary components--the first temperature measuring element, the heating element and the second temperature measuring element--can be applied thermally practically directly onto the corresponding inside faces of the measuring instrument housing, the flow monitor according to the teachings of copending application Ser. No. 07/172,457 is thermally markedly fast-reacting. Since in this flow monitor it is also unnecessary to pot the functionally necessary components--the first temperature measuring element, the heating element and the second temperature measuring element--with an epoxy resin that is a good thermal conductor, this flow monitor can also be manufactured with less effort and expense. However, such heat transmission measuring instrument is still somewhat complicated and expensive from the manufacturing standpoint.
Accordingly, it is an object of the present invention to provide for a heat transmission measurement device which is simpler to manufacture and less expensive.